A. Field of the Invention
The present invention relates to a method and apparatus for image calibration for an optical scanner, and especially to a method and apparatus which can automatically calibrate a skew image caused by the oblique displacement of the image sensor in an optical scanner.
B. Description of the Prior Art
The optical mechanism for a scanner uses an adjustment jig to tune the horizontal level, right and left positions, and focus for an image. The optical mechanism also includes a calibration paper for a charged couple device to read, thereby to calibrate scanned images. However, the optical mechanism is subjected to damages caused by improper use or handling in shipment. Consequently, the optical mechanism will shift and generate skew images as shown in FIG. 1. Refer to FIG. 1, the skew image indicated by coordinates of As(i,j) is in contrast to a normal coordinates indicated by coordinates of A(i,j). The front edge of the skew image can be detected by gray levels. For instance, in FIG. 1, the skew black line indicates the front edge of the skew image.
When the optical mechanism of a scanner is damaged, the image quality cannot be improved simply by fine-tuning. Moreover, there are various possible reasons for scanner to generate skew images. It would be very difficult to find a universal method for solving the occurrence of skew images for every scanner. Even if the scanner is sent back to customer service for repair, there is no guarantee that the occurrence of skew images will never happen again.